1. Field of the Invention
The present invention relates to a vibration energy absorber device for decreasing a vibration force acting on a construction, by absorbing vibration energy produced by an earthquake or the like, and for preventing the construction from being damaged or destroyed and, more particularly, to a device for absorbing vibration energy by utilizing plastic deformation of structural elements.
2. Discussion of Background
Conventional energy absorber devices of this type can be classified into three types, according to their energy absorption mechanisms. A conventional energy absorber device of a first type uses a fluid or viscoelastic material. When vibration energy acts on this material, it is converted to a viscous flow of the material, thereby absorbing the vibration energy. A conventional energy absorber device of a second type comprises overlying metal members. When vibration energy acts on these metal members, it is converted to a frictional force produced between the contact surfaces of the metal members and can thus be absorbed. A conventional energy absorber device of a third type comprises a plastically deformable member. When vibration energy acts on the member, the energy is absorbed by plastic deformation of the member.
Among these energy absorber devices, the device of the third type, i.e., the plastic deformation type for absorbing vibration energy by utilizing plastic deformation of the material, has a simpler structure as compared to the other energy absorber devices, and can be manufactured at low cost.
Typical examples of a conventional plastic deformation type vibration energy absorber are cyclic shear energy absorbers described in U.S. Pat. Nos. 4,117,637 and 4,499,694. Each prior art device comprises a pair of fixing members respectively fixed on the lower surface of the construction and the upper surface of the foundation, with an elastoplastic member being located between the fixing members. The ends of the elastoplastic member are coupled to the corresponding fixing members.
According to the conventional energy absorber having the elastoplastic member, if vibration energy produced by an earthquake or the like acts on the construction and both the fixing members are displaced, relative to each other, in the horizontal direction, the elastoplastic member is cyclically shear-deformed upon this relative displacement of the fixing members. That is, part of the vibration energy is consumed due to plasticity of the elastoplastic member. In other words, part of the vibration energy is absorbed by the elastoplastic member and therefore, vibration energy directly acting on the construction can be decreased. The construction can thus be effectively protected from the vibration energy produced by an earthquake or the like. When the elastoplastic member is cyclically shear-deformed, the diameters of both end portions of the elastoplastic member radially decrease; conversely, the diameter of the intermediate portion of the elastoplastic member, i.e., the axial central portion thereof radially increases. Since both ends of the elastoplastic member receive cyclic loads by this extension and contraction, they are subject to repeated extension and contraction deformations. The extended ends of the elastoplastic member, however, cannot be restored to their original state, even after a compression force has acted thereon. For this reason, if the distance between the fixing members is constant, the end portions of the elastoplastic member must radially decrease in order to absorb the axial extentions of the end portions thereof and the material of elastoplastic member flow from the end portions to the axial central portion. As a result, the elastoplastic member is deformed as described above. Upon drawdown of the ends of the elastoplastic member, the resistance of the elastoplastic member against rupture is decreased, and hence, the vibration energy absorption capacity of the elastoplastic member is degraded. In the worst case, the elastoplastic member ruptures at the drawdown portion.
In order to solve the problem posed by the elastoplastic member itself, energy absorber devices having elastoplastic members and metal coils wound around respective elastoplastic members have been proposed in the above-mentioned official gazette. This coil allows plastic shear deformation of the elastoplastic member itself, and prevents the elastoplastic member from being radially contracted or expanded. When the elastoplastic member is shear-deformed, the coil must be deformed along with the elastoplastic member. However, since the coil is made of a continuous wire rod, a torsional force as well as a tension force acts on this wire rod. The coil then receives the torsional force as well as the restriction force (acting in a direction perpendicular to the longitudinal direction of the wire rod) for restricting radial deformation of the elastoplastic member. Therefore, the coil tends to be damaged. In order to prevent damage to the coil, the diameter of the wire rod of the coil can be increased, so as to improve the mechanical strength of the coil. However, since the rigidity of the coil itself is therefore increased, shear deformation of the elastoplastic member is restricted by the coil. Therefore, vibration energy cannot be effectively absorbed by the elastoplastic member.
In the energy absorber device described in U.S. Pat. No. 4,499,694, the elastoplastic member and the coil are housed in a rubber bearing for supporting the construction. It is therefore difficult to properly maintain the elastoplastic member and the coil. Since the vibration energy absorption capability is degraded upon repetition of shear deformation, the elastoplastic member must be periodically inspected and replaced if necessary. In addition, the coil must also be inspected for damage to the wire rod thereof. However, since the elastoplastic member and the coil are housed in the rubber bearing, inspection and replacement cannot be easily performed. In particular, the need for replacement of the elastoplastic member is often inaccurately judged.